Automatic gain control



Jun@ 2y i936. W. VAN B. ROBERTS AUTOMATIC GAIN CONTROL Filed oct. 31, 1929 w mmv [NVENTOR WALER VAN E. ROBERTS Hill ATTORNEY Patented June 2, 1936 UNITED STATES AUTOMATIC GAIN CONTROL Walter van B. Roberts,

Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application october 31, 1929, serial No. 403,6s1

7 Claims.

The present invention relates to radio receiving systems, and more particularly to a method of, and means for, maintaining the intensity of received signals at a uniform level.

As is well known, radio signalling Waves are subject to variable attenuation. In fact, the intensity of received signals vmay vary so greatly as to make it difficultl at times, sometimes even impossible, to obtain satisfactory reception. Climatic and solar conditions often cause these variations. `More rapid changes in reception intensity are produced by a well known phenomenon termed fading.

Slow changes of the flrst kind may be compensated by manual adjustment of the sensitivity of the receiver, but compensation of the rapid fading effects necessitates some type of automatic adjustment. Methods of maintaining a uniform reception level have been proposed in the past, as 20 for example by Affel in United States Patent 1,468,687 issued lSeptember 25th, 1923. In these methods any change in the attenuation of the medium is immediately countered by an opposite change in the gain characteristic of the receiving system.

As is well known, in most radio communication systems, and especially in vradio telephony, the signal is transmitted as a modulated carrier wave, which may be considered asa wave of single frequency and varying amplitude, or as a group of waves including a constant wave of the carrier frequency and side waves representing the signal. y NowI have invented and devised an improved means for maintaining constant the intensity of the carrier voltage reaching the detector, in spite of variations in the intensity of carrier voltage picked up by the antenna. My method of automatic gain control has been particularly applied to a receiving system of the double detection or superheterodyne typein which an intermediate frequency carrier wave is produced by demodulationof the incoming wave with an auxiliary wave locally generated.

The superheterodyne system lends itself very .readily to the practice of the invention for reasons which will appear later.

Accordingly, it is one ofthe main objects of my present invention to provide a heterodyne receiving system in which the intensity of the carrier input to the detector is maintained at a uniformlevel, by amplifying the intermediate frequency wave prior to the stage of detection, and ,varying .the amount ofl this amplification in an ,OPDQSt Sense. t0. and .in synchronism with. vari- (Cl. Z50-20) ations of the intensity of the carrier voltage picked up by the antenna.

Another important object of the invention is to provide in a superheterodyne receiving system, a method of maintaining the receiver output at a level bearing a constant relation tothe sound intensity level at the transmitter microphone, which comprises heterodyning the signal energy with local energy to produce energy of intermediate frequency, variably amplifying a portion of this intermediate frequency and then rectifying it to produce audible signals, amplifying another portion of the intermediate frequency energy by means of a separate amplifier of iixed amplifying power, rectifying it in a separate rectifier, and employing the direct current component of this rectified energy for varying the amplification of the aforesaid variable amplifier in an inverse manner as the amplitude of the signal carrier current received by the antenna varies. y

Other objects of the invention arel to improve generally the efficiency of automatic gain control devices, and particularly to provide a superheterodyne receiver with an automatic gain control which is durable and reliable in operation and secures exact regulation. Y e w e The novel features whichIbelieve to be characteristic of my invention are set 'forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood .by reference to the -following description taken in connection with the drawing in which I have indicated diagrammatically lseveral l circuit organizations whereby my invention may be carried into effect.

In the drawing,

Fig. 1 diagrammatically shows a superheterodyne receiving system embodying an automatic gain control according to the,y invention,

Fig. 2 shows a modification of the invention applied to a tuned radio frequency set.

Referring to the accompanying drawing in which like characters of reference indicate the same parts in the different figures, the receiving system in Fig. 1 is of the double detection or superlieterodyne type in which an intermediatefrequency carrier wave is produced by heterodyning the incoming signal wave with an auxiliary wave locally generated. The incoming waves are collected by the grounded antenna A, and amplified by one or more` stages of tuned radio frequency .ampliers in a manner well known to those skilled .in the art.,A v

The heterodyne, or auxiliary wave is supplied by a local oscillator which is coupled, as at M, to the grid circuit of the frequency changer device l. The latter is preferably of the screen grid type of space discharge devices. Thus, the amplified incoming signal energy is fed into the input circuit of the frequency changer l as the local energy is fed thereto, with the subsequent production of intermediate frequency waves in the output circuit of the changer.

The intermediate frequency is fed to an amplier by means of transformer coupling M1, the mutual inductance of Which is variable. The output of the amplifier is detected in any Well known manner, if used for radio telephony, and utilized in any desired fashion, as by phones, loud speaker, etc.

In the anode circuit of the frequency changer tube I there is disposed a coil 2, in series with the variable coil of coupling M1, the coil 2 being inductively coupled to the coil 3 in the input circuit of an amplier tube 4. The tube 4 is a space discharge tube of the screen grid type. The amplified output of the tube 4 is transferred by means of an inductive coupling M3 to the input circuit of a detector tube 5, which tube is a space discharge tube of the triode type.

The output circuit of the detector tube 5 includes a low pass filter circuit embodying inductance L and capacity C. There is connected in series with the inductance L a winding 6, which is wound about a portion of the periphery of a split ring armature 'l made of iron or other magnetizable ferrous materials.

Thus, when current flows through the winding 6 the annular armature 'l is magnetized, the opposite points of the split periphery functioning as opposite poles of a magnet. A movable coil 8 connected in series with a source of direct current 9 through suitable slip rings (not shown) is mounted between the poles of the armature 1 1n such a manner that it will rotate when there is a change in intensity in flow of current through the winding 6.

The coil 8 is mechanically connected in any desired manner, shown by dotted lines I0, to the variable coil Il of the coupling M1. In other words, the coils 8 and Il are so designed and mechanically connected as to give any desired fraction of maximum input to the intermediate frequency amplifier corresponding to each possible value of the incoming carrier current strength.

It will thus be seen that as the intensity of the incoming signal energy varies, a device, which I designate by the term pilot, will function to employ the direct current component of the rectified intermediate frequency Waves for varying the efficiency of transfer of intermediate frequency energy to the intermediate frequency amplifier. Specifically, this is accomplished by varying the coupling M1 in synchronism with, but in an opposite sense to, variations in incoming carrier intensity.

The operation of the system disclosed should be obvious from the aforementioned detailed description. To recapitulate, however, filtered direct current from the pilot equipment rotates the coil 8, the latter being energized by direct current from the source 9, and the coil 8 is mechanically connected to the variable primary Il of the coupling M1 so that the input to the intermediate frequency amplifier is determined by the direct current output of thepilot, and

hence by the carrier strength of the incoming signal. It is to be understood, of course, that any other type of mobile device may be employed in place of the split ring armature 'l and moving coil 8, and for this reason I broadly designate the latter device as a direct current operated motor device.

It is, also, to be understood that the motor device could vary the degree of coupling between the frequency changer and the intermediate frequency amplifier by means other than a mechanical connection. Again, it is within the scope of my disclosed arrangement to employ the pilot motor device for varying the efficiency of amplification of the intermediate frequency waves by affecting a portion of the intermediate frequency circuit other than the coupling between the frequency changer and the intermediate frequency amplifier.

In Fig. 2 I have disclosed another mode of securing the same result, as achieved by the arrangement shown in Fig. 1, without the employment of any moving parts, and have applied it to a tuned radio frequency type of receiver. In this modification the bias on the control electrode of the space discharge tube employed as a radio frequency amplifier and hence the amplication thereof, is determined by the direct current output of the pilot equipment, the latter being connected in this modification directly to the signal energy collecting means, whereby the bias of the said control electrode is determined by the incoming carrier intensity. As in Fig.

1, the grounded antenna A has coupled to it a tuned radio frequency amplier shown for simplicity as consisting of a single stage of amplification, the amplified output of the latter being impressed upon the input circuit of a detector and then utilized in any well known fashion, as by phones, loud speakers, etc.

In this modified arrangement the pilot equipment includes the same funamental devices as in the arrangement in Fig. 1. That is to say, the pilot amplifier tube 4 and its attendant circuits are broadly designated as pilot amplifier in Fig. 2, and the input circuit of the latter is directly coupled as at M3 to the grounded antenna circuit A. The output of the pilot amplifier is coupled to the input of the pilot detector, which p is, in this case, a two element rectifier tube 5 shown in conventionalized form in Fig. 2. The rectified output of the pilot detector is filtered, as in Fig. 1, by means of a capacity C and a choke coil L1.

In the modified arrangement, however, the direct current component of the rectified output of the tube 5' is passed through a resistance R, the latter being tapped at two points 2l and 22. 'I'he point 2| is connected to the grid circuit of the amplifier tube 20, while the tap 22 is connected to the filament of the latter tube.

It will thus be seen that variations in potential drop across the resistance R will result in variations in bias of the control electrode of amplifier tube 20; further, that this bias variation will occur in synchronism with, but in an opposite sense to, Variations in the incoming carrier current intensity.

It will now be evident that the reason for the earlier statement that the invention is particularly adapted to superheterodynes is that in a superheterodyne the same radio frequency amplifier, operating at constant amplification, and frequency changer may be used to supply both the EIO ordinary intermediate amplifier and the pilot intermediate amplifier, whereas when the invention is applied to a tuned radio frequency receiver it is advisable to vary the amplification of all the radio frequency tubes to obtain good control of amplification, thus making the pilot system a completely independent radio receiver which has to be tuned for different stations.

It is of course possible to use a single amplifier of fixed amplification to feed both the ordinary detector and the pilot detector, and let the output of the pilot detector control the coupling between the radio frequency amplifier output and the ordinary detector, or control volume by any other means subsequent to the radio frequency amplifier, but this is likely to allow overloading of some of the radio frequency tubes on strong signals.

A feature of my invention is that not only can adjustment be made such that weak stations come in with the same volume as strong ones, but it is even possible to so adjust that weak stations come in with greater volume than stronger ones, a feature not found in the prior volume control systems where there is only one detector and its output only increases the gain of the receiver when the volume is below normal. The best the prior systems could do is to prevent so much discrepancy occurring between the volume obtained from strong and Weak stations, or during fading, as would occur in the absence of any automatic volume control at all.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications in the circuit arrangements, as well as in the apparatus employed, may be made Without departing from the scope of my invention as set forth in the appended claims.

What I claim iszl. In combination, means for collecting signal energy, means for combining the latter with local energy to produce intermediate frequency energy, means to detect the latter, and pilot means, responsive to predetermined variations in the intermediate frequency energy level, to control the efficiency of energy transfer between the said combining means and the said detector means.

2. In combination, means for collecting signal energy, means for combining the latter with local energy to produce intermediate frequency energy, means to detect the latter, and pilot means, responsive to predetermined variations in the intermediate frequency energy level, to control the coupling between the said combining means and the said detector means.

3. In combination, means for collecting signal energy, means for combining the latter with local energy to produce intermediate frequency energy, means to amplify the latter, and pilot means, responsive to predetermined variations in the intermediate frequency energy level, to control the coupling between the intermediate frequency amplifying means, and the combining means.

4. In combination, means for collecting signal energy, means for combining the latter with local energy to produce intermediate frequency energy, means to detect the latter, and pilot means, including an amplifier, rectifier and motor member responsive to predetermined variations in the intermediate frequency energy level, to control the efficiency of energy transfer between the said combining means and the said detector means.

5. In combination, means for collecting signal energy, means for combining the latter with local energy to produce intermediate frequency energy, means to detect the latter, and pilot means, including an amplifier, rectifier and motor member responsive to predetermined variations in the intermediate frequency energy level, to control the coupling between the said combining means and the said detector means.

6. In a superheterodyne receiving system the steps in a method of maintaining the receiver output at a uniform level which comprise, heterodyning the signal energy with local energy to produce therefrom energy of a predetermined intermediate frequency, detecting a portion of the whole of the produced intermediate frequency energy to derive the desired signals, rectifying another portion of the intermediate frequency energy produced and employing the direct current component of the resulting rectified energy for` determining the portion of the whole of the intermediate frequency energy to be detected to produce the desired signals.

7. In a superheterodyne receiving system the steps in a method of maintaining the receiver output at a predetermined uniform level which comprise, heterodyning the signal energy with local energy to produce therefrom energy of a predetermined intermediate frequency, detecting a portion of the whole of the resultant intermediate frequency energy to produce the desired signals, rectifying another portion of the resultant intermediate frequency energy and employing the direct current component of the resulting rectified energy for determining the portion of the resultant whole of the intermediate frequency ergy.

WALTER VAN B. ROBERTS. 

